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The ghost in the machine

Henry Fox Talbot, the pioneering photographer and amateur etymologist, struggled with the word "sublime". He complained that, "The great stumbling block in this word [is] that it apparently contains the preposition SUB, while the sense requires SUPER." These days, we rather like the idea of the limen, or perceptual threshold, and the liminal is often invoked in art as a moment of weightless suspense. Perhaps it's more useful to think of it as Fox Talbot's stumbling block. In attempting to look over from under, one might trip up.

Frank Tallis is trying to look under from over, that is, to know what is by definition the unknown. His lucid and agile account begins with St Augustine's acknowledgement that, "I cannot grasp all that I am." While Tallis cannot solve this conundrum, he illuminates it by mapping the maps, charting the ways in which ideas about the unconscious have evolved out of, as well as in reaction to, one another.

Tallis begins with Leibniz, who is credited with bringing unconscious mental processes into philosophical debate, and who wrote his seminal work in response to John Locke. From there, Tallis takes us on a colourful journey through Romanticism, Darwinism, Freud, surrealism, Turing, computing, quantum mechanics and neuroscience, to show that the more we learn about the mind, the more bound up with the body it appears to be. Neuroscientists are now attempting to locate Freudian phenomena in the brain.

An incidental but equally fascinating map is that of the analogue. As Tallis says, "In many respects, the history of psychology has been shaped by the selection of metaphors." This concise work may not be the place to pursue the implications of this observation, but they are intriguing. Freud used the analogy of Dutch land reclamation, in which the land of the ego is threatened by the waters of the id. Jung saw minds as an archipelago; Plato as a chariot drawn by two opposing horses. In a somewhat overextended metaphor, Tallis has Freud as a psychological grease monkey, "loosening nuts and bolts; feeling for parts; probing the gears; warming his palms on the dynamo".

Many agreed that one had to switch off the conscious in order to reach the unconscious through hypnosis, mental breakdown, drugs or sleep. For Freud, dreams were "the royal road", and this idea of setting out after our-selves is affirmed in physiological studies which demonstrate that we rationalise post hoc. When it comes to thoughts, says Tallis, "We are not aware of their assembly, only their delivery." We explain ourselves after the event.

The great mapmakers produced all sorts of models of strata, process, machinery and flow. By the 1960s, the computer model had become dominant, and the study of thought increasingly scientific. One approach is to find out how something works by looking at what happens when it doesn't. Freud's idea of mental schism as a result of childhood trauma has been literally borne out, as brain scans have shown that, in traumatised patients, the bridge between the two hemispheres (the corpus callosum) is reduced. One young patient had a left side that wanted to be a draughtsman and a right side that wanted to be a racing driver.

The breakdown of cognitive function resulting from brain damage can be remarkably specific, so it has helped doctors to locate what happens where. Tallis outlines syndromes in which the patient believes all friends are strangers, or vice versa, the problem of the alien hand or the incomplete act, as in combing only half your hair.

Another way of breaking into the unconscious is to interrupt it through subliminal interventions such as Pavlovian conditioning. In a notorious case, two American teenagers shot themselves after listening repeatedly to a Judas Priest album that was said to contain subliminal messages encouraging suicide. Certain amnesiacs continue to learn even though they are not conscious of doing so, bringing to mind, as Tallis says, Huxley's notion of "hypnopaedia": children being educated as they slept. There are patients who have received subliminal therapy and think they have cured themselves.

Evolutionary theory has come up with some of the most engaging explanations for subconscious behaviour because it acknowledges that the survival instinct has been socialised. Self-deception is a necessary subconscious tool with which we disguise our true motives. Insincerity is easily picked up from voice and expression, but these giveaways can be overcome when we believe ourselves, too.

Primo Levi noted that,"When running through a list of names of minerals, one is confronted by an orgy of personalities." Here, we have Leibniz designing a submarine and helping to set up a bank; Galvani probing his frog during an electrical storm; Mesmer leaching off his rich wife; the would-be scrupulous Darwin and the gloriously neurotic Freud.

Plagiarism and misattribution lead Tallis to propose an alternative canon. A 19th-century surgeon, James Esdaile, used hypnotism in thousands of operations and lost only 16 patients at a time when the death rate was closer to 50 per cent. His achievement was ignored because he worked in India. Darwin procrastinated over his Origin of Species until a young upstart called Alfred Russel Wallace sent him a paper on the same subject. Freud never properly acknowledged his debt to Pierre Janet and, in later life, refused to meet him. On the other hand, Leibniz courteously shelved his critique of Locke, who had just died, while Breuer delayed publication on theory that preceded Janet and let Freud continue without him.

All these stories are colourful enough and the one irritation in this book is Tallis's occasional lapse into an ingratiating slanginess: Coleridge "was into drugs"; Leibniz "was no slacker"; "In the ancestral environment, it didn't pay to be cool."

Tallis is a psychologist and neuroscientist, and so this history concentrates on managing and mending. His conclusive analogy is that "identity is to the brain what the shape of a wave is to sea water", begging the question: who or what is magnetic pull?

We are still trying to answer this obliquely. Freud's TheInterpretation of Dreams (1900) took eight years to sell 600 copies, but still does brisk business. Trilby, The Strange Case of Dr Jekyll and Mr Hyde and The Picture of Dorian Gray are embedded in our culture. However much the mind is unravelled into electrical impulse, we still see it as "a nightmare world of supernatural presences, rotting flesh, and violent sexual passion", as Tallis describes Schoenberg's opera of the unconscious, Erwartung.

Freud's claim to have made the "third blow" to mankind, after Copernicus and Darwin, frames the book. Now, evolutionary theory, neuroscience, psychology and artificial intelligence research have reinforced the dominance of the unconscious and Freud's reputation is being dusted off. Tallis warns of the "narcissism of exaggerating our mystery", but he also acknowledges that what we need now, as with quantum mechanics, is a philosophical response. He ends with an excited conjecture about the future redefinition of human nature, setting aside the human limitations that this book does so much to reinforce. We might accept that consciousness is not an executive homunculus or site in the brain but an "emergent property", and that the self is a "local phenomenon" long ago rejected in eastern thought, but they remain what we are aware of.

Metaphor, the great tool, is too contingent and is itself liminal, a point between observation and articulation. It moves us on via the familiar, a case of two steps forward, one step back. To borrow that of Fox Talbot, we are still at the stumbling block, perhaps now not even knowing which way we are looking, down or up.

Lavinia Greenlaw is a poet and critic. Her novel Mary George of Allnorthover is out in paperback (Flamingo)